Coaxial line to wave guide matching section



March 21, 1950 L. P. HUNTER 2,501,335

COAXIAL LINE T0 WAVE GUIDE MATCHING SECTION Filed Dec. 27, 1947INVEN'fOR Lloyd P.Hunter.

ATTORN Patented Mar. 21., 1950 CSAXIAL LINE T WAVE MATCHING SECTIONLloyd 1?. Hunter, Qak Ridge, Tenn.,. assignor to.

Westinghouse Electric Corporation, East Pittsburgh, Pa, a corporation ofPennsylvania AppiicationDecember 27, 1 947,.SerialaNo; 794,148:

5 Claims ((31; 178-44) My invention relates to junctions of coaxialcables towave-guides for transmitting ultra-highfrequencyelectromagnetic energy, and in particular relates to a novel dispositionof the coaxial cable relative to. the wave guide to which connectionismade.

It has been conventional in the prior art to effect a junction between acoaxial cable carrying ultraehigh-frequency electromagnetic energy andawave guide through: which the energy is to flow by arranging the axisof the coaxial cablenormal to thecentral axis of thewave guide at thejunction, the end of the-wave guide near the junction being closed by aconducting wall, and the central, core. of the wave guide usually being.positioned. in the. neighborhood of a quarter wave length distant from.this partition. The axis ofthe coaxial cable is conventionally normal:to the surface of. the wave guide atits point: of entry; where the waveguide is a rectangular cross section, the coaxial cable passes throughthe middle ofone wall of the guide.

I- have found that while, insuch conventional. junctions, thecharacteristic impedance of the wave guide may be sufiicientlywellmatched to that of the coaxial cable by methods well known in theart, the arrangement is usually quite frequency sensitive so that if itis desired to transmit a fairly broad band of frequencies it isnecessary to resort to alterations of the shape of the junction, such asthe provision, of tuning devices which make the arrangement complicated"and difficult to manufacture. It is also frequently awkward from thestandpoint of space limitations to be compelled to have the axis of the.coxial cable normal to the axis of the wave guide.

One object of my invention is accordingly: to provide-a novel way ofconnecting a coaxial cable to awave guide which shall avoid the awkwardspace limitation imposed by junctions of'the'prior art for suchelements.

Another object of my invention is to provide a junction between acoaxial cable and a wave guide which is less sensitive to frequencyvariations of the electromagnetic energy being transmitted,

through them than were junctions of the prior art.

Still another object of my invention is to provide a new form ofjunction between a coaxial cable and a wave guide which shall have arelatively broad frequency acceptance while at the same time preservinga voltage standing wave ratio at least as good as those of junctionsconventional in the prior art for such elements.

Still another object of my invention is to pro- 2 vide a-novel type of;junction between a coaxial cable and a wave guidewhich shall be ofsimple formand. easy manufacture while at the same time preserving atransmission efiiciency equal to'junctions between such elements of theprior art type. I

Still another object of my invention is to provide'a formof junctionbetween a coaxial cable and awaveguide. which: shall be relativelyinsensitive to frequencyrvariations while; at the same time itis.easyto: manufacture andfree from the necessity for providing complicatedadjustable tuning devices.

Other objects of my invention will become; apparent upon reading thefollowing description; taken in connection withv the drawings, inwhichthe single-figure showsan embodiment of my invention suitable forjoining a" coaxial cable. of circular section with a wave guideofrectangular section. 7

Referring, in detail to the drawings, a wave guide- I- which: may havethe rectangular crosssection which iswell known in the wave guide art,comprises parallel-conducting side wallslandlikewise a parallelconducting top and bottom wall. Thewidth of this: guide may; forexample, betwic'ethe height, although it" is to be understood. that myinvention isnot limited to employment. with:-dimensi0ns. of this:particular ratio. While such a wave guide is capable of transmittingelectromagnetic; energy in. a. number of different modes of? vibrationwhich are well knownto'thoserskilled inthe art, I will; mention as anexample of theseta: mode of vibration. in which the electriavectorsextendverti'cally between the topandbottom'wallsand varysinusoidall'y indensity distribution, both parallel to the axis of theguideand'zinistherdirection transverse to'the guide. Such: a. mode: of vibration issom'etimes referred toiasa'a-TEartyp'e ofwave transmission.

The wave guide 1 has a conducting end wall 2 perpendicular to its axiswhich is provided with acentral circula opening of the same size as theinternal diameter of the sheath 3 of a concentric line supplying energyto the guide. The sheath 3 makes a tight abutment with the end wall 2.The central core 4 of the concentric line extends in substantialcoincidence with the cen tral axis of the guide I into the interior ofthe latter for some distance and then turns at right angles in the planeof the electric vector to make contact with the face of the guide. Ihave found that a good match between the impedance of the concentricline and the guide results if the distance between the above mentionedpoint of attachment of the core to the guide wall from the end wall 2 isabout one-half of the wave length of the electromagnetic energy beingtransmitted. I have likewise found it desirable for many purposes toprovide a diaphragm extending part way across the guide I at a shortdistance beyond the end of the core 4 of the coaxial line, thisdiaphragm 5 being one form of inductive diaphragm which may beproportioned to match out any small irregularities and reflectionsintroduced into the transmitted waves by the junction of the coaxialline and the wave guide. Other types of inductive diaphragm may beemployed for this purpose where desirable.

To give a specific example, I once used a rectangular wave guide havinga width of 2.84" and a height of 1.34", inside dimensions, and made ofcopper which was connected as I have described above to a coaxial linehaving a sheath of brass which is 1 /32 inches in inside diameter and acore of brass of A; of an inch diameter. When transmitting in the abovementioned TEu,1 mode of vibration electromagnetic Waves having anopen-air wave length of 10.6 cm., the system was operated over a rangeof frequency and I found that the standing wave ratio in the wave guideremained between 1.1 and 1.0 for a 5% change in frequency, while thestanding Wave ratio in the coaxial line remained within the limits 1.03and 1.00. The core of the coaxial line in these tests projected 3.25inches into the wave guide from the end wall 2, and the diaphragm 5,which was 8.76 inches from the end of the guide I, extended from top tobottom of the guide adjacent one wall and had a width of 0.437 of aninch.

I claim as my invention:

1. In an electromagnetic wave transmitting system, a rectangular waveguide having an end wall and a coaxial transmission line joining saidwave guide with its central core parallel to the central axis of thewave guide at their junction, one end of its sheath being in abutmentwith said end wall of said guide, and its central core extending intosaid guide for a distance about equal to one-half wave length of thewaves traversing said coaxial line and being connected with a side wallof said wave guide whereby the impedance of said wave guide issubstantially matched to the impedance of said coaxial transmissionline.

2. In combination, a rectangular wave guide having an end wall and acoaxial transmission line joining said wave guide with its central coreparallel to the central axis of the wave guide at their junction, oneend of its sheath being in abutment with said end Wall of said guide,and its central core extending into said guide for a distance aboutequal to one-half wave length corresponding to a mode of Wave vibrationwhich said guide is capable of transmitting, and making contact with aside wall of said guide whereby the impedance of said wave guide issubstantially matched to the impedance of said coaxial transmissionline.

3. In combination, a rectangular wave guide having an end wall and acoaxial transmission line joining said wave guide with its central coreparallel to the central axis of the wave guide at their junction, oneend of its sheath being in abutment with said end wall of said guide,and its central core extending into said guide for a distance aboutequal to one-half wave length corresponding to the TEc,1 mode oftransmission of said guide and making junction with a side wall of saidguide whereby the impedance of said wave guide is substantially matchedto the impedance of said coaxial transmission line.

4. In combination, a rectangular wave guide having an end wall and acoaxial transmission line joining said wave guide with its central coreparallel to the central axis of the wave guide at their junction, oneend of its sheath being in abutment with said end wall of said guide,its central core extending into said guide for a distance about equal toone-half wave length corresponding to the TEn,1 mode of transmission ofsaid wave guide and making junction with a side wall of said Wave guidewhereby the impedance of said wave guide is substantially matched to theimpedance of said coaxial transmission line, and an inductive diaphragmin said wave guide beyond its junction with said core.

5. In combination with a rectangular wave guide having a transverse endwall, a coaxial line having one end of its sheath joined to said endwall of said guide, and having its core extending through said end wallparallel to the central axis of said wave guide and extending beyondsaid end wall for a distance about equal to a half-wave length of theTEUJ mode of transmission of waves through said guide and turningparallel to the electric vector of said waves and making juncture with aWall of said guide whereb the impedance of said wave guide issubstantially matched to the impedance of said coaxial transmissionline, and an inductive diaphragm means in said wave guide beyond thesaid juncture.

LLOYD P. HUNTER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,151,118 King Mar. 21, 19392,232,179 King Feb. 18, 1941 2,316,151 Barrow Apr. 13, 1943 2,398,095Katzin Apr. 9, 1946 2,432,093 Fox Dec. 9, 1947

